1. Field of the Invention
The present invention relates to an optical filter in which long-period gratings (LPGs) are formed in an optical waveguide. Unlike a short-period grating reflecting light of a specific wavelength, the long-period grating is grating converting light of a specific wavelength among core-mode light propagating in a confined state in the core region, into cladding-mode light and radiating the cladding-mode light to the outside of the cladding region, for example, as disclosed in U.S. Pat. No. 5,703,978.
2. Related Background Art
An optical waveguide (e.g., an optical fiber) in which refractive index modulation having the period of several hundred xcexcm (long-period grating) is formed in an optical waveguide region, converts light of a specific wavelength among the core-mode light propagating in a confined state in the core region of the optical waveguide, into cladding-mode light and radiates the cladding-mode light to the outside of the cladding region. Namely, the optical waveguide with the long-period grating formed therein acts as an optical filter having wavelength selectivity. This optical filter is characterized by nonreflective nature, as apparent from the aforementioned loss producing mechanism. Therefore, the optical filter is suitably applied to uses to attenuate the core-mode light of a specific wavelength with no reflection, and is suitably applicable, for example, as a gain equalizer for equalizing gains of an optical amplifier in wavelength division multiplexing (WDM) optical communications.
An optical filter with a normal long-period grating of uniform period formed in the core region of the optical waveguide demonstrates a transmission characteristic having the shape of the Gaussian function in the wavelength band of 100 nm and having only one loss peak, as shown in FIG. 9. However, the optical filter used as the above-stated gain equalizer or the like is required to have a complex transmission characteristic as shown in FIG. 10. In order to meet this request, the optical filter as shown in FIG. 11 was proposed. This optical filter is produced in the following manner. Namely, first prepared are a plurality of optical waveguides (three in the figure). Each of the optical waveguides has a long-period grating and has margins at both ends of the grating. Then each of the long-period gratings is housed in a package, the optical waveguides are fusion-spliced between their margins, and the spliced portions are reinforced by a reinforcement. A transmission characteristic of this optical filter is superposition of transmission characteristics of the respective long-period gratings.
A conceivable reason for this is as follows. Namely, the optical filter shown in FIG. 11 is usually coated with resin 40 around the margins for the purpose of reinforcement, as shown in FIG. 12. Therefore, the cladding-mode light, into which the core-mode light has been converted in the front long-period grating LPG-a, is radiated through the resin 40 to the outside and is not recoupled with the core-mode light in the rear long-period grating LPG-b. For this reason, as shown in FIG. 13, the transmission characteristic of the optical filter (indicated by L3) results in superposition of the transmission characteristic of the long-period grating LPG-a (indicated by L1) and the transmission characteristic of LPG-b (indicated by L2). The transmission characteristic shown in FIG. 13 is that of the optical filter wherein in the long-period grating LPG-a the period of refractive index modulation is 361 xcexcm and the grating length 13 mm and wherein in the long-period grating LPG-b the period of refractive index modulation is 356 xcexcm and the grating length 15 mm. However, the optical filter in the structure wherein the margins are fusion-spliced as shown in FIG. 11 is of large size and it is thus difficult to house the entire filter in a package.
In order to make the whole filter compact enough to be housed in a package, it can be contemplated that the optical filter is constructed by forming a plurality of long-period gratings in tandem in a unitary optical waveguide. In this configuration, there is no need for provision of the margins for fusion splicing and thus the whole of the optical filter can be made compact. However, the transmission characteristic of the optical filter of this structure is different from a desired one, which is superposition of transmission characteristics of the respective long-period gratings in the unitary optical waveguide.
A conceivable reason for this is as follows. Supposing two long-period gratings LPG-a and LPG-b are formed in tandem in a unitary optical waveguide to constitute an optical filter, as shown in FIG. 14, the cladding-mode light, into which the core-mode light has been converted in the front long-period grating LPG-a, will be recoupled with the core-mode light in the rear long-period grating LPG-b. For this reason, the transmission characteristic of this optical filter becomes complex and different from superposition of transmission characteristics of the respective long-period gratings LPG-a and LPG-b, as shown in FIG. 15, and prediction of the characteristic is not easy. Each of the long-period gratings LPG-a and LPG-b herein is of structure similar to that shown in FIG. 13, and the spacing between them is 2 mm.
In contrast with it, Document xe2x80x9cM. Harumoto et al., xe2x80x9cCompact long-period grating module with multi-attenuation peaks,xe2x80x9d Electron Lett., Vol. 36, No. 6, pp.512-514 (2000)xe2x80x9d describes the optical filter that can be formed in compact size and that can readily implement a desired transmission characteristic. The optical filter described in this Document is one wherein two long-period gratings LPG-a and LPG-b5 are formed in tandem in a unitary optical waveguide, as shown in FIG. 16. In this optical filter, a mode number of cladding-mode light to be coupled with core-mode light in the long-period grating LPG-a is different from that in LPG-b5 in the waveguide band used in communications.
For example, suppose in the front long-period grating LPG-a the period of refractive index modulation is 361 xcexcm and the grating length 13 mm, in the rear long-period grating LPG-b5 the period of refractive index modulation is 415 xcexcm and the grating length 14 mm, and the spacing between them is 2 mm. In this configuration, the mode numbers of the cladding-mode light to be coupled with the core-mode light in the communication wavelength band (1525 nm to 1565 nm; so-called C-band) are six in the front long-period grating LPG-a and five in the rear long-period grating LPG-b5. Therefore, the sixth cladding-mode light, into which the core-mode light has been converted in the front long-period grating LPG-a, is rarely recoupled with the core-mode light in the rear long-period grating LPG-b5. Therefore, as shown in FIG. 17, the transmission characteristic of this optical filter (indicated by L1) becomes nearly equal to superposition of transmission characteristics of the two long-period gratings LPG-a and LPG-b5 (indicated by L2).
The inventors conducted research on the above prior art and found the following problem. Specifically, FIG. 17 also shows the transmission characteristic of the configuration with the spliced portion between the two long-period gratings LPG-a and LPG-b5 (indicated by L2) and a difference between them (indicated by L3), in addition to the transmission characteristic of the configuration with the two long-period gratings LPG-a and LPG-b5 formed in tandem in the unitary optical waveguide (indicated by L1). As seen from this figure, though the mode numbers of the cladding-mode light to be coupled with the core-mode light in the C-band are made different from each other between the two long-period gratings LPG-a and LPG-b5, the transmission characteristic of the configuration with the two gratings formed in tandem in the unitary optical waveguide is different by about xe2x88x920.25 dB to +0.32 dB in the C-band from the transmission characteristic of the configuration with the spliced portion between the two gratings.
As described above, the transmission characteristic of the optical filter described in aforementioned Document is also different from the superposition of the transmission characteristics of the respective long-period gratings LPG-a and LPG-b5, and prediction of the characteristic is not easy. For example, in the case of a long-haul optical communication system the transmission distance of which is even several thousand km, the difference can adversely affect the operation of the system and result in degradation of communication characteristics.
The present invention has been accomplished in order to solve the above problem and an object of the invention is to provide an optical filter that can be constructed in compact size and that can readily implement a desired transmission characteristic.
An optical filter according to the present invention is an optical filter for attenuating light of a predetermined wavelength in a communication wavelength band. This optical filter comprises a unitary optical waveguide having a core region and a cladding region, and a plurality of long-period gratings formed in the optical waveguide. In first and second long-period gratings selected from the plurality of long-period gratings, a difference is not less than 100 nm between a wavelength at which optical coupling becomes maximum between core-mode light and cladding-mode light of a predetermined mode number in the first long-period grating and a wavelength at which optical coupling becomes maximum between core-mode light and cladding-mode light of the same mode number as the cladding-mode light of said predetermined mode number, in the second long-period grating. More preferably, the above difference is not less than 200 nm. This allows the optical filter to be constructed in compact size and to readily implement a desired transmission characteristic.
In the optical filter according to the present invention, at least one of the first and second long-period gratings can have a phase rotating portion. This configuration makes it easier to implement a desired transmission characteristic.
In the optical filter according to the present invention, the communication wavelength band includes a C-band (1525 nm to 1565 nm) and an L-band (1565 nm to 1610 nm), a wavelength at which optical coupling is maximum in the communication wavelength band between the core-mode light and cladding-mode light in the first long-period grating lies in the L-band, and a wavelength at which optical coupling becomes maximum in the communication wavelength band between the core-mode light and cladding-mode light in the second long-period grating lies in the C-band. This optical filter can be suitably applied to a wavelength division multiplexing optical communication system utilizing both the C-band and the L-band.
The present invention can be further fully understood from the detailed description and accompanying drawings which will follow. It is to be considered that these are presented simply for illustration of the invention and do not limit the present invention.